Chalk-browed mockingbird description

During Charles Darwin’s voyage aboard the Beagle, the eminent naturalist was clearly taken by a bird he described in Uruguay as “possessing a song far superior to that of any other bird in the country” (3). This bird was the chalk-browed mockingbird, a small, ubiquitous passerine, with a conspicuous white supercillium, or ‘eyebrow’, sandwiched between a blackish eye stripe and a dark grey crown (2)(4). The upperparts of its body are predominately greyish brown, while the flight feathers and the long semi-erect tail are blackish, with white tips. Below, the throat and belly are greyish white, but the flanks are often streaked with darker markings. Across the chalk-browed mockingbird’s broad range, four subspecies, which exhibit minor differences in body size, bill length and plumage, are currently recognised: Mimus saturninus saturninus, M. s. frater, M. s. arenaceus, and M. s. modulator(2).

Related species

Chalk-browed mockingbird biology

The omnivorous diet of the chalk-browed mockingbird consists of various insects, spiders, worms, fruit, seeds and berries, and occasionally the eggs and nestlings of other birds (2). Although this mockingbird forages mainly on the ground, it will regularly perch in low bushes or trees, especially during the breeding season when the males spend long periods singing (4). Breeding occurs from September to January with each monogamous pair often accompanied by several helpers that assist with territorial defence, nest-guarding and feeding of young. A clutch of three to four eggs is laid in a small, loosely constructed nest and incubated for 12 to 15 days. After hatching the young are confined to the nest for another 12 to 15 days, and are fed by the parents for around a week after fledging. The juveniles then remain in their parental territory through the non-breeding season and potentially as helpers over the subsequent breeding season (2). The nests of the chalk-browed mockingbird are commonly parasitized by shiny cowbirds which lay their own eggs in the nests of the mockingbirds (2)(5). This typically results in the mockingbird unwittingly incubating the eggs of the shiny cowbird and rearing its young, often to the detriment of its own (5)(6).

Chalk-browed mockingbird range

The chalk-browed mockingbird occurs in southern Suriname and in large parts of central, southern and northeastern Brazil, west into Bolivia and Paraguay, and south into Uruguay and northern Argentina (2)(4).

Species with a similar range

Chalk-browed mockingbird habitat

Found in a wide range of habitats including savannas, low woodland, bushland, palm swamp, pasture and urban parks and gardens (2). For the most part, it is only absent from densely forested areas and, at the other extreme, areas completely devoid of trees (2)(4).

Chalk-browed mockingbird status

Chalk-browed mockingbird threats

Despite considerable nesting losses due to parasitism by the shiny cowbird, the chalk-browed mockingbird is very common through most of its wide range and is not believed to be under significant threat (2).

Authentication

Glossary

Incubated

The act of keeping eggs warm so that development is possible.

Parasitized

In general, a relationship between two species in which one benefits at the expense of the other.

Passerine

A group of more than 5,000 species of small to medium-sized birds which have widely varied plumage and shape. They all have three toes pointing forward and one directed backward which assists with perching, and are sometimes known as perching birds or song birds.

Subspecies

A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.

Darwin, C.R. (1845) Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2nd edition. John Murray, London. Available at: http://darwin-online.org.uk

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